Method and apparatus for clarifying green liquor

ABSTRACT

In a chemicals recovery plant in a sulphate pulp mill, in order to provide green liquor with improved sedimentation and filtration properties, there is passed, from a flow (10) of slaked lime and green liquor between a lime slaker (9) and a causticizing tank (1 1), a partial flow (32, 35) to a soda dissolver (2) or to a container (26) for unclarified green liquor provided after the soda dissolver. For this purpose there is provided, in a line (10) between the lime slaker (9) and the causticizing tank (11), a branch line (32, 35) which passes to the soda dissolver (2) or to a container (26) for unclarified green liquor provided after the soda dissolver.

BACKGROUND OF THE INVENTION

The present invention relates to a process and a device in a chemicalsrecovery plant in a sulphate pulp mill.

From SE-B-8700549-2 there is known a process for treating green liquor,more particularly in such a way that the green liquor's content ofparticularly inorganic impurities and soot obtains better sedimentationand filtration properties and, owing to that, can be separated from thegreen liquor and accordingly removed from the chemicals cycle in asimpler and more effective way than before. This is achieved by addingto the non-clarified green liquor, while stirring quick lime comprisingfrom 0.5 to 10%, preferably from 1 to 3% of the amount of quick limenecessary for complete causticizing of the green liquor.

According to SE-B-8700549-2 the quick lime is added either to a sodadissolver, in which melt from a soda recovery unit is dissolved in weakliquor, and from this to a mixing tank or directly to the mixing tank.From the mixing tank the green liquor formed is passed to a filter, fromwhich solid particles are removed for dumping whereas clarified greenliquor is passed to a lame slaker. From the lime slaker the mixture oflime and green liquor is passed to a causticizing tank from which themixture of liquor and lime sludge formed is passed to a filter. Thefiltrate formed therein, white liquor, is passed to the digester housewhereas the material filtered off, the lime sludge, is passed to a limesludge silo. From this the lime sludge is passed to a washing filterfrom which the weak liquor formed is added to the above-mentioned sodadissolver whereas the lime sludge is passed into a lime kiln where it isburnt to lime. The quick lime is passed to the lime slaker as well as,according to the above, to either the soda dissolver or the mixing tank.

SUMMARY OF THE INVENTION

The object of the present invention is to simplify and cheapen the knownprocess and owing to that also make a simplified and considerable morecheap device possible. The invention is based upon the understandingthat it is the slaked lime formed in the soda dissolver and in themixing tank, respectively, that is active in the process in order toobtain the desired result. According to the present invention it istherefore suggested that the addition of lime is in the form of alreadyslaked lime, that is the slaked lime that is already present in theprocess and that is formed in the lime slaker. Thus, from the flow oflime and green liquor to the causticizing tank there is taken a partialflow which is passed to the mixing tank or alternatively to the sodadissolver. This is done before the causticizing process has beencompleted, i.e. that uncausticized liquor (lime milk) is passed into themixing tank.

By using in this way slaked lime that is already present in the plant,the expensive and long conveyors, which, in plants according toSE-B-87005492, are required to convey slaked lime to the soda dissolverand the mixing tank respectively, are avoided. Instead there is obtaineda closed system for lime circulation from the lime slaker through aline, provided for this purpose, to the mixing tank, alternatively thesoda dissolver, from the mixing tank to the green liquor filter and thenback to the lime slaker.

On the apparatus side there is required for this only that there isprovided, from the existing line between the lime dissolver and thecausticizing tank, a branch line to the mixing tank and the sodadissolver, respectively, optionally together with a pump, a pump vesseland means for regulating the partial flow withdrawn.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail in the following withreference to the enclosed schematic drawing showing an example of achemicals recovery plant in a sulphate pulp mill set up to carry out theprocess of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the drawing 1 designates a running shute which passes, from a sodarecovery unit not shown, melt to a soda dissolver 2 in which there is astirrer 3. In the soda dissolver 2 also a line 4 for weak liquorconcludes. From the soda dissolver 2 a line for green liquor passes to agreen liquor clarifier 6, from the bottom of which sediment can beremoved through a line 7. From the green liquor clarifier 6 a line 8 forclarified green liquor passes to a lime slaker 9, from which a line 10for slaked lime and green liquor passes to a causticizing tank 11. Aline 12 connects the causticizing tank 11 with a white liquor filter 13,from which on one hand a line 14 passes carrying filtered white liquorto a digester house not shown and, on the other hand, a line 15 carryingfiltered-off material (lime sludge) to a lime sludge silo 16. From thisa line 17 passes to a washing filter 18, in which the lime sludge iswashed with hot water entering the washing filter through a line 19. Inthe washing filter the weak liquor formed is passed through the line 4previously mentioned to the green liquor clarifier 6 whereas the limesludge is passed, through a path 20, into a lime kiln 21 for burning toquick lime. The quick lime is passed through a line or conveyor 22 to alime silo 23, from which quick lime is withdrawn to the lime slaker 9.For so-called draining of the lime cycle the lime slaker 9 is providedwith a drainage 24 for inert substances. All lines mentioned above areshown in the drawing with solid lines.

As an alternative to passing, through line 5, the green liquor from thesoda dissolver 2 to the green liquor clarifier 6, the green liquor maybe passed, through a line 25, to a mixing tank 26 provided with astirrer 27 and from this through a line 28 to a green liquor filter 29,from which filtered-off material (sludge)is passed for dumping through aline 30, whereas filtered green liquor is passed, through a line 31connected to line 8, to the lime slaker 9. The alternative lines 25, 28and 3i are shown with dash dotted lines.

The plant described above is a conventional one. According to the priorart of SE-B-8700549-2 the conventional plant has been supplemented inthat alternative lines or conveyors pass from the line or conveyor 4 inorder to introduce quick lime into to the soda dissolver 2,alternatively the mixing tank 26.

In order to save the considerable plane and operation costs for theselines or conveyors, it is suggested according to the present invention,as mentioned above, that slaked lime (lime milk) is taken from the flowin line 10 from the line slaker 9 to the causticizing tank 11. For thispurpose there is provided in line 10 a branch line 32, which passes,through an optional pump jar 33 with the subsequent pump 34 and a line35, the lime milk to the soda dissolver 2--in case a green liquorclarifier 6 is used--or through a line 36 to the mixing tank 26--in casesuch a one is used. In case any pump is not required to transport thelime milk, the line 32 may be connected directly to line 35 through aline 37.

Means, which are not shown, for regulating the partial flow withdrawnfrom line 10, e.g. 3% of the total flow, are of course provided in lines32 and 37 (comprising control means for the pump 34) and in line 37,respectively, such as valves and other metering means making sure thatthe addition of slaked lime to the soda dissolver and the mixing tank,respectively, will be as intended.

Through the invention there has been provided an "inner" cycle in theform of a completely closed system which differs from the conventional"outer" lime cycle as well as from the lime cycle known fromSE-B-8700549-2 with drainage to the soda dissolver or the mixing tankand which as such does not require any replenishment of lime because ofdrainage, since the necessary amount of lime in the form of slaked limeis continuously taken from the outer lime cycle through line 32.

We claim:
 1. A process for producing green liquor with improvedsedimentation and filtration properties in a chemicals recovery plan ofa sulphate pulp mill comprising:introducing weak liquor and melt into asoda dissolver in order to form unclarified green liquor; directing aflow of unclarified green liquor from the soda dissolver to a clarifierto remove solid particles from the unclarified green liquor so as toproduce clarified green liquor; directing a flow of clarified greenliquor from the clarifier to a lime slaker; introducing a supply of limeinto the lime slaker along with the clarified green liquor from theclarifier; directing a flow of slaked lime from the lime slaker to acausticizing tank; and re-directing a partial portion of the flow ofslaked lime, prior to the causticizing tank, to one of the sodadissolver and a mixing tank interposed in the flow path of green liquorfrom the soda dissolver to the lime slaker to improve the removal ofsolid particles from the unclarified green liquor, wherein the partialportion of the flow of slaked lime corresponds to approximately 3% ofthe total flow from the lime slaker to the causticizing tank.
 2. Achemical recovery system in a sulphate pulp mill for producing greenliquor with improved sedimentation and filtration propertiescomprising:a soda dissolver into which weak liquor and melt areintroduced and processed so as to form green liquor; a green liquorclarifier; a lime slaker; a first flow line in flow communication fromsaid soda dissolver through said green liquor clarifier to said limeslaker; means for introducing lime into said lime slaker; a causticizingtank; a second flow line in flow communication from said lime slaker tosaid causticizing tank; and a branch line means for re-directing apartial portion of the flow of slaked lime, prior to said causticizingtank, to said soda dissolver.
 3. The system according to claim 2,wherein said branch line means comprises a pump.
 4. The system accordingto claim 3, wherein said branch line means comprises a pump jar.
 5. Achemical recovery system in a sulphate pulp mill for producing greenliquor with improved sedimentation and filtration propertiescomprising:a soda dissolver into which weak liquor and melt areintroduced and processed so as to form green liquor; a green liquorfilter; a lime slaker; a first flow line in flow communication from saidsoda dissolver through said green liquor filter to said lime slaker; amixing tank provided in said first flow line between said soda dissolverand said green liquor filter; means for introducing lime into said limeslaker; a causticizing tank; second flow line in flow communication fromsaid lime slaker to said causticizing tank; and a branch line means forre-directing a partial portion of the flow of slaked lime, prior to saidcausticizing tank, to said mixing tank.
 6. The system according to claim5, wherein said branch line means comprises a pump.
 7. The systemaccording to claim 6, wherein said branch line means comprises a pumpjar.